Obtaining dithiols by the reaction of an alkyl dihalide with an alkaline salt of H.sub.2 S, directly or via a bis-thiocyanate, bis-thiourethane or bis-thioacetate comprises several stages. The yields from reactions leave something to be desired. See N. KARASH, Organic Sulfur Compounds, vol. 1, pp. 199-207 (1961).
The photochemical process which comprises reacting H.sub.2 S with an olefin in the presence of a catalytic system comprising a benzophenone or thiobenzophenone and a trialkyl phosphite gives excellent results with monoolefins. See French Pat. No. 2,424,907 and European Pat. No. 60,754. Unfortunately, for dienes, secondary reactions occur which spoil the yield of the desired dithiol (formation of polymers by interaction of the dithiol with the diene). To limit these secondary reactions, particular operating conditions have been recently disclosed in French Patent Application No. 86/17,640. The aim is to have a very large excess of hydrogen sulphide in the reaction medium.
Another access route to dithiols, particularly to 1,4-butanedithiol from butadiene, is described in U.S. Pat. No. 3,069,472. The reaction scheme disclosed in this patent comprises a free radical or photochemical addition of a tertiary-alkyl mercaptan to butadiene to form a 1,4-di-(tertiary-alkylthio)butane which, by splitting in the presence of a suitable catalyst, gives 1,4-butane-dithiol. This splitting reaction provides, as a by-product, the olefin corresponding to the structure of the original tertiary mercaptan. This olefin is not directly reusable in a cyclical process and is moreover susceptible to oligomerization upon splitting.
A process has now been found whose principal advantages are a much higher yield and selectivity of dithiols, the absence of by-products of the dithiol such as polysulphides, and the simultaneous regeneration of one of the compounds leading to the original product.
The preceding references are hereby incorporated by reference.